In power inverters for feeding electric energy from a DC power source into an AC power grid, it is known to galvanically isolate the AC power grid from the DC power source by means of a DC/DC converter. Such a DC/DC converter is connected upstream of a DC/AC converter of the inverter and galvanically isolates a DC voltage link at its input from a DC voltage link between the DC/DC converter and the DC/AC converter, for example by means of a high frequency transformer or by means of a high frequency AC voltage link with galvanically isolating capacitors. A further converter stage, like for example a boost or buck converter, may be provided between the DC power source and the DC voltage link at the input of the DC/DC converter. Typically, the capacitance of the DC voltage link at the input of the galvanically isolating DC/DC converter is smaller than the capacitance of the DC voltage link between the galvanically isolating DC/DC converter and the DC/AC converter, because a smaller buffer capacitance is sufficient for supplying the high frequency AC voltage link of the DC/DC converter as compared to supplying the inverter bridges of the DC/AC converter. When the inverter is switched off, however, the large capacitance of the DC voltage link between the DC/DC converter and the DC/AC converter is to be discharged within a defined interval of time in a controlled way for security reasons.
For the purpose of feeding on-board voltage rails of power inverters including the inverters described above, it is known to have an on-board supply unit which generates the required auxiliary voltages via a flyback converter out of a DC voltage which is present in a DC voltage link of the inverter. It is preferred to feed the on-board voltage rails out of a DC voltage link which is already loaded by a connected DC power source even prior to starting the inverter. It is also known to use the AC voltage of a connected AC power grid as an alternative power source for the on-board power supply.
EP 1 107 438 A2 discloses a balancing circuitry for two partial capacitances of a DC voltage link of a power inverter, which are connected in series. In this balancing circuitry the partial voltages at the partial capacities are each used for feeding one primary winding of a flyback converter, which is used as an on-board supply unit feeding on-board voltage rails of the inverter.
EP 2 058 921 A1 discloses a power inverter for feeding electric energy from a DC power source into an AC power grid. Here, a charger/discharger circuit is arranged between a DC voltage link at the input of a DC/AC converter of the power inverter and an accumulator to load this accumulator out of the DC voltage link and to pre-charge the DC voltage link out of the accumulator if needed. Further, the accumulator serves as a secondary power source of an on-board power supply of this known inverter.
There still is a need for a power inverter for feeding electric energy from a DC power source into an AC power grid, which comprises optimized on-board power supply.